The present invention relates to the art of ink ejection and more particularly to a method and apparatus for controlling the return flow of the ink in the nozzle of an ink ejector head used in a printer in which ink is ejected in the form of drops or droplets directly onto the paper or other print carrier.
More particularly, the present invention relates to an arrangement having an ink ejector head incorporating an ink flow system which itself has a chamber capable of being supplied with ink from an ink reservoir and a nozzle through which the ink is ejected, and a pressure producer capable of being subjected to pulses for applying pressure to the ink in the chamber so as to cause the ink to be ejected through the nozzle.
In such ink ejector heads, pressure pulses generally occur when the pressure is generated to drive the ink out of the nozzle, which pulses propagate not only toward the nozzle but also in a direction away from the nozzle so that they strike regions from whence they are reflected. The reflected pressure pulses or pressure waves lead to the formation of improperly shaped drops. The drop formation is also influenced by the geometry of the ejection system, the arrangement of the energy flow channels, and the surface configuration of the nozzle and of the chambers. When the pulse is turned off, the pressure producer snaps back to its rest position and this creates a sudden vacuum or reduction of pressure in the ejection system and consequently in the nozzle, and this, in turn, results in rapid return flow of the ink into the nozzle. This not only sucks air from the atmosphere into the nozzle, but also influences the umbilical cord connecting the drops with the quantity of ink flowing back into the nozzle until a drop breaks off, so that secondary or so-called after-drops are formed from the umbilical cord and/or the main drop. These after-drops and the main drop are propelled toward the print carrier and often move at a high velocity different from that of the main drop.
One type of conventional droplet ejector system, such as is shown in German laid-open patent application (Offenlegungsschrift) No. 2,405,584 corresponding to U.S. Pat. No. 3,832,579 issued Aug. 27th, 1974) has an ink drop ejector, a ceramic oscillator attached thereto, an ink inlet and a nozzle for forming the drop. The energy flow channels are within a disc made of pressure absorbing material, which disc additionally has an absorber channel whose length is sufficient to eliminate waves during the generation of pressure. The reflection of waves, and even the reflection of multiple waves, can be eliminated by providing an appropriately appropriated, dimensioned so-called transition zone which includes the absorber channel, the pressure chamber and the outlet channel up to the region of the nozzle. It has been found, however, that the relatively great pressure reduction in the absorber system reduces the amount of energy required for the drop formation which must be compensated for by increasing the pulse voltage for the ceramic oscillator. Moreover, the pressure wave propagation pattern will differ from ink to ink, so that the ejector system has to be specially designed for every different type of ink, to say nothing of the fact that higher voltages across the ceramic oscillator increase the costs of the electronic equipment.
It is, therefore the object of the present invention to control the return flow of ink during the suction phase initiated by the end of the pressure pulse in such a manner that no after-drop is formed or if one is formed, that it will be accelerated to the velocity of the main drop.